The present invention relates to a fan filter unit and to a vertical laminar-flow type clean room including one or more of such fan filter units (such as the clean rooms used in manufacturing semiconductor devices).
In clean rooms, fan filter units serve two main purposes. First, the fan filter units serve to filter dust and other small particles from the clean room air. Second, because a clean room may contain gases that may be hazardous to the health of workers or damaging to the processes being carried out in the clean room, the fan filter units serve to filter unwanted gases from the clean room air. For example, in a clean room in which a semiconductor manufacturing line is constructed, various reagents such as hydrofluoric acid and hydrochloric acid are used in the chemical stations arranged along the semiconductor manufacturing line. Although treatment systems are installed to neutralize any reagent gases escaping into the air, the treatment systems are not completely effective, and trace amounts of the reagent gases still escape from the chemical station into the work areas in the clean room without being treated. In addition to reagent gases, some organic gases are given off by the construction materials in the clean room. Moreover, nitrogen oxides such as NO.sub.x and sulfur oxides such as SO.sub.x are contained in the fresh air circulated in the clean room from the outside. Further, various toxic gases are generated by the corrosion of glass fiber by hydrofluoric acid in the high-performance filters used in the clean room. Ammonia gas also evaporates from the bodies of the workers. These hazardous gases cause chemical contamination of the air in the clean room. This chemical contamination circulating in the clean room may be hazardous to the health of the workers and may deteriorate the film quality of semiconductor devices in certain steps of semiconductor processes.
FIGS. 4(a) and 4(b) illustrate two types of conventional vertical laminar-type flow clean rooms and their circulation systems. In FIG. 4(a), there is shown a clean room 1, a free-access floor 1a of the clean room 1, semiconductor manufacturing facilities 2 mounted on the floor 1a of the clean room 1, a dust removal filter 3 (such as a high performance HEPA filter or a ULPA filter) distributed over the ceiling of the clean room 1, an air conditioning unit 4 located outside of the clean room 1, a ventilation unit 5 for air circulation, and gas absorption filters 6. The gas absorption filters 6 are located in ventilation ducts that lead from underneath the floor 1a of the clean room 1 to the air conditioning unit 4 and ventilation unit 5. In FIG. 4(a), air is circulated downward from the dust removal filter 3 through the clean room to the free access floor 1a, through the gas absorption filters 6, air conditioner unit 4 and ventilator unit 5, and back to the dust removal filter 3. The air conditioner unit 4 has an intake for fresh air as well as for the circulating air of the clean room.
The second type of circulation system shown in FIG. 4(b) is similar to that shown in FIG. 4(a), except that instead of a separate dust removal filter 3 and ventilation unit 5, the system in FIG. 4(b) has a plurality of fan filter units 7 arranged along the ceiling of the clean room. The fan filter units 7 incorporate both functions of the dust removal filter 3 and the ventilation unit 5.
FIG. 5(a) is a cross-sectional view of a conventional fan filter unit used in the circulation system of FIG. 4(b). As illustrated in FIG. 5(a), each fan filter unit 7 includes a ventilation fan 7c, a dust removal filter 7b, a unit casing 7c (housing the ventilation fan 7c and dust removal filter 7b), and a gas absorption filter 6 housed in an air intake duct 7d on the side wall of the unit casing 7c.
FIG. 5(b) is a cross-sectional view of another conventional fan filter unit. In the fan filter unit 7 of FIG. 5(b), a gas absorption filter 6 is mounted on the upper surface of a unit casing 7c such that the gas absorption filter 6 covers an air intake 7d of a ventilation fan 7c.
The foregoing conventional fan filter units pose several problems in terms of the removal of hazardous gases from the clean room work area, the layout of the fan filter units in the clean room, and the ease of maintenance of the fan filter units.
First, when the centralized-type clean room as shown in FIG. 4(a) is divided into a plurality of clean work areas A, B and C, as illustrated by dashed lines in the figure, and different steps of the semiconductor process are conducted in the different clean room work areas, a variety of hazardous gases with varying properties are produced depending on the reagents used in the steps of the semiconductor process performed in the different work areas. Since the gas absorption filter 6 is inserted in the ventilation paths common to the clean room work areas A, B and C, a type of gas absorption filter selected for removing one type of gas (for example, either an alkaline gas, an oxide gas, or an organic gas) may not be suitable for removing other kinds of hazardous gases produced from the other work areas.
Second, in the distributed type of clean room as shown in FIG. 4(b), the gas absorption filter 6 is mounted in the air intake 7d of the unit casing 7c of each fan filter unit 7. Due to this arrangement, it is necessary to space the adjacent fan filter units 7 apart from each other and to secure spaces between the adjacent fan filter units 7 for clearance of the air intake ducts and for the maintenance of the gas absorption filters. Therefore, the arrangement of the fan filter units 7 is limited, and it is difficult to add more fan filter units.
Third, in the configuration of the fan filter unit shown in FIG. 5(b), although the space limitation described above is not present, time and manpower are required to remove the gas absorption filter 6 covering the upper face of the unit casing 7c for the inspection of the ventilation fan 7c housed in the fan filter unit 7. In addition, in the configuration of the fan filter unit shown in FIG. 5(b), because of the placement of the gas absorption filter 6 on the upper face of the fan filter unit 7, it is hazardous for workers to walk on the unit casing 7c during maintenance operations of the fan filter unit 7. Therefore, the costs of the clean room facilities soar, since it is necessary to provide a separate path above the fan filter units for their maintenance.